This invention relates to arc coating processes and devices such as are disclosed in U.S. Pat. Nos. 3,625,848 and 3,836,451 to Alvin A. Snaper and U.S. Pat. Nos. 3,783,231 and 3,793,179 to L. Sablev, et al. These systems are characterized by high deposition rates and other advantageous features. However, these advantages can be somewhat offset due to instability of the arc. That is, the arc involves currents of about 60 amperes, or more, concentrated into a cathode spot so small that current densities are 10.sup.3 to 10.sup.6 amperes per square inch. The voltages are 15 to 45 volts. Thus, power densities at the tiny cathode spot are in the order of megawatts/inch.sup.2. Accordingly, local violence is an understatment. The target surface under the cathode spot flash evaporates from the intense heat. It is this evaporated target material which deposits as the coating on a substrate. The cathode spot migrates about the target surface in a random, jerky motion with reported velocities of many meters per second. Because of this random movement, damage to the device and contamination of the coating can occur if the spot moves off the target surface.
Different solutions to the arc instability problem have been proposed. Thus, in Sablev, et al., U.S. Pat No. 3,793,179, a shield is placed close to the edge of the target. In particular, it is placed at a distance from the target which represents less than a mean free path of the gas present. In an arc discharge, gas and plasma are generated at the cathode spot with sufficient violence that local mean-free-paths may occasionally be reduced to a few thousandths of an inch. When such a blast of local high pressure is blown under the shield, which is spaced at several millimeters (.about.80 thousandths of an inch), there is finite possibility the arc can migrate under the shield. When this happens, there will be arc damage to the cathode, contamination of the evaporant, or the arc will extinguish.
Sablev, et al. U.S. Pat. No. 3,783,231 (copy submitted herewith) apparently addresses the foregoing problem by providing a feedback mechanism of some complexity that emphasizes the frustrations caused by the problem. The feedback involves the utilization of a magnetic field to retain the cathode spot on the target surface. U.S. Pat. No. 2,972,695 (copy submitted herewith) to H. Wroe also suggests the utilization of a magnetic field for cathode spot retention.
A problem related to that of arc stabilization is the need for ultra-clean conditions for the target assembly. That is, the target surface usually initially includes contaminants such as oxides and the like. Since the oxides emit copious amounts of electrons, the arc initially prefers to locate itself at such sites until the oxide contaminant is evaporated away. Only after all such sites are evaporated can meaningful evaporation of the target commence during the target evaporation phase. During this initial cleaning phase, the arc may move onto the back and sides of the cathode where, as discussed above, it may damage the structure or evaporate contaminating metals into the chamber.
As will be described hereinafter, certain embodiments in accordance with the present invention are subject to the above-discussed problems during the initial cleaning phase. However, once this phase is completed, control of the arc becomes quite complete in these embodiments during the target evaporation phase. Nevertheless, steps must be taken to avoid the problems arising during the initial cleaning phase.